A seat cushion pad disposed in a seating section of a seat of an automobile or the like has been composed of an under-buttock section, an under-thigh section, side bulging sections on both sides of the seating surface, and a rear end section, and has been produced by injecting an urethane formulation stock solution, which is produced by mixing a polyol component formulation solution and an isocyanate component, into a mold, followed by foam molding. Likewise, a seat back pad disposed in a backrest section has been composed of an upper backrest section, a lower backrest section, side bulging sections on both sides thereof, an upper end section, and a lower end section, and has been produced in a similar manner.
In the seat cushion pad and the seat back pad for a vehicle, sections in direct contact with the buttock section and the back of an occupant, for example, the under-buttock section of the seating surface, are applied with most of the occupant's weight, and materials for these sections have a close bearing on the seat comfort and ride comfort. Therefore, the constituent materials of the under-buttock section and the like are important from the view point of the seat comfort and the ride comfort. However, the other sections are applied with a small load of the weight and, therefore, the importance is relatively low.
In order to facilitate weight reduction and cost reduction of the seat cushion pad for a vehicle, various improved seat cushion pads for a vehicle have been proposed, in which, for example, an under-buttock section of a seating surface having a largest influence on the seat comfort has been made a high-density section, and the other sections have been made low-density sections.
Japanese Unexamined Patent Application Publication No. 2002-153357 has proposed a seat cushion pad for a vehicle in which a part of or entire seating surface except an under-buttock section has been made to have a density lower than the density of the under-buttock section and a hardness nearly equal to the hardness of the under-buttock section.
It has been proposed that “the 25% hardnesses” defined in JASO-B408 are allowed to agree with each other in order to match the feeling (feeling in seating) about the under-buttock section and the feeling about the other sections with each other. However, in the method for measuring the 25% hardness, a large disk having a diameter of 200 mm is used and, in addition, a value of reaction force is read after the 25% compression is kept for 20 seconds. Therefore, even when the 25% hardnesses are allowed to agree with each other, the feeling about the foam of the under-buttock section is different from the feeling about the foam of the other sections, because different formulations are used. Consequently, the discomfort is produced between the feelings.
As an example thereof, FIG. 10a shows load-deflection diagrams and stress relaxation after keeping for 20 seconds during measurement of the 25% hardness of two types of urethane foam having an equal 25% hardness of 314 N. FIG. 10b is a diagram showing a magnified stress relaxation portion.
As shown in FIGS. 10a and 10b, even when the 25% hardnesses are equal, the amounts of stress relaxation are different and the reaction forces of load-deflection curves before keeping for 20 seconds are different depending on the formulations.
In order to attain the same 25% hardness, a resin in the urethane foam having a low-density urethane formulation (C) must be made harder. In general, a harder resin exhibits a larger amount of stress relaxation. Therefore, the reaction force before keeping for 20 seconds of the urethane foam having a low-density formulation (C) is higher than that of the urethane foam having a high-density formulation (D) by about 7 N. As a result, at the instant when being pushed, the urethane foam having the formulation C produces a harder feeling.
With respect to the low density, high hardness urethane, the resin itself is harder than the resin of the high-density urethane having the same level of hardness. Therefore, the tension of the pad surface portion is high and a taut and hard feeling is produced as compared with a soft, elastic, and high-density urethane. Even when the 25% hardnesses are equal, the difference therebetween becomes evident by pressing the foam with a finger or the like having a small area.
FIG. 11 shows a load-deflection diagram when the formulation C and the formulation D having an equal 25% hardness are pressed with a disk having a diameter of 10 mm. The low-density, hard formulation C exhibits a compressive load (reaction force) about 40% higher than that of the high-density formulation D. This indicates that since such a difference is exhibited by pressing with the disk having a diameter close to the thickness of a finger, the formulation C feels harder to the touch with a hand even when the 25% hardnesses are equal.
The thighs and hands rather than the buttock are mainly brought into contact with the under-thigh section and side bulging sections in seating. These sections have contact areas smaller than that of the under-buttock section, and loads to be applied are also small. Therefore, pressing with a smaller disk rather than pressing with a disk having a diameter of 200 mm is suitable for matching the feeling about them with the feeling about the under-buttock section because a produced feeling is close to an actual feeling.
Likewise, in the case where these two types of urethane foam are pressed with a pressure probe 131 at the front end of a push-pull gauge 130 shown in FIG. 12a, the values of the formulation C and the formulation D are 17.5 N and 13.7 N, respectively. Therefore, the formulation C exhibits a higher value and becomes harder. Since this pressure probe 130 is pressed into a sample by 15 mm so as to measure the reaction force, as shown in FIGS. 12(b) and 12(c), the front end side of the pressure probe 131 is in the shape of a hemisphere having a diameter of 20 mm.
As described above, in the case where different materials are used for the under-buttock section and the other sections, e.g., the under-thigh section and the side bulging sections, the feelings about the foams cannot be matched with each other simply by allowing the 25% hardnesses defined in JASO-B408 (JIS K 6401) to agree with each other, and uncomfortable feelings are produced among the formulations.